---
title: "HOST-SSH-004 SSH Offers Weak Cryptography"
canonical: https://openvapt.com/checks/HOST-SSH-004.html
html: https://openvapt.com/checks/HOST-SSH-004.html
site: OpenVAPT, https://openvapt.com/
license: OpenVAPT Source-Available Licence 1.3, https://openvapt.com/legal/licence-text.html
modified: 2026-10-14
version: 0.1.0-alpha.6
---

# HOST-SSH-004 SSH Offers Weak Cryptography

Checks the SSH ciphers, MACs, key exchange and host key algorithms for outdated choices.

- Domain: Server
- Severity: Medium
- Control category: Cryptography and TLS
- Fix effort: low
- CVSS v3.1 vector: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N`
- CIS Controls v8: 3.10
- NIST CSF 2.0: PR.DS-02
- CWE: CWE-327
- OWASP Top 10 2021: A02:2021
- NIST SP 800-53 Rev. 5: SC-8, SC-13
- Benchmark topic: SSH server configuration: approved ciphers, MACs and KEX (topic only, no benchmark text)

## Why It Matters

CBC-mode ciphers, RC4, MD5/SHA-1 MACs and 1024-bit Diffie-Hellman have known practical weaknesses. Offering them lets a network attacker downgrade or weaken the session.

## How It Detects

Reads ciphers, macs, kexalgorithms and hostkeyalgorithms from sshd -T and flags any algorithm whose name contains a known-weak fragment, plus the SHA-1 based ssh-rsa host key signature algorithm.

## How to Fix

Remove the listed algorithms. On OpenSSH 8.x and later the defaults are already strong: delete any custom Ciphers/MACs/KexAlgorithms lines, or set explicit modern lists such as  Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com  then reload sshd.

## How to Verify

```
sshd -T | grep -Ei '^(ciphers|macs|kexalgorithms)'  should list no cbc, arcfour, md5 or sha1 entries, and hostkeyalgorithms should not list ssh-rsa.
```

## Learn the Theory

- [P2 SSH Security](https://openvapt.com/academy/p2.md)

## References

- [OpenBSD, sshd_config(5) manual page](https://man.openbsd.org/sshd_config)
- [RFC 9142, Key Exchange Method Updates for SSH](https://www.rfc-editor.org/rfc/rfc9142.html)
- [NIST SP 800-131A Rev. 2, Transitioning Cryptographic Algorithms](https://csrc.nist.gov/pubs/sp/800/131/a/r2/final)

## Source Code

https://github.com/OpenVAPT/openvapt/blob/574f7f3333c44b75a3c649e2d93a76d507f75adf/internal/checks/host/ssh_weak_crypto.go

Run it: `openvapt scan --only HOST-SSH-004`. Read it offline: `openvapt explain HOST-SSH-004`.

---

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